1. Field of the Invention
The present invention relates generally to a method and apparatus for removing interference from an UpLink (UL) receive signal in a time division duplex system. More particularly, the present invention relates to a method and apparatus for removing a DownLink (DL) signal of an adjacent Base Station (BS) which causes interference to a UL signal of a BS in a Cross Time Slot (CTS) of a flexible time division duplex system.
2. Description of the Related Art
Typically, a Time Division Duplex (TDD) system time-divides the same frequency resource to transmit and receive data. A conventional TDD system has a fixed ratio of DownLink (DL) and UpLink (UL) of a Base Station (BS) as shown in FIG. 1A. Recently, as the traffic distribution varies according to a type of the wireless data service, a flexible TDD system is being developed to regulate the ratio of the DL and the UL according to the traffic distribution as shown in FIG. 1B.
In a conventional TDD system with a fixed UL/DL ratio, interference is caused by a DL signal of a neighbor BS using the same frequency resource in a DL section where a BS sends a signal to a Mobile Station (MS) as shown in FIG. 2A. In a UL section where the MS sends a signal to the BS, interference is caused by a UL signal of an MS belonging to the neighbor BS using the same frequency resource as shown in FIG. 2B.
In addition to the interference in the UL and DL sections in the flexible TDD system, interference is caused in a Cross Time Slot (CTS) having the different UL/DL ratio between neighbor cells.
FIG. 3 illustrates interference in a CTS in a conventional flexible TDD system.
In the flexible TDD system of FIG. 3, a UL signal of an MS B 305 in the boundary of the neighbor cell acts as interference 312 to a signal 310 transmitted from a BS A 301 to an MS A 303, and a DL signal of the neighbor BS A 301 acts as interference 316 to a signal 314 transmitted from the MS B 305 in the cell boundary to the BS B 307.
As above, it is hard for the BS and the MS to detect the original signal because of the interference in the CTS. More particularly, since the transmit power of the BS is greater than that of the MS and the antenna gain and the channel condition of the BS are much better than those of the MS, the DL signal of the neighbor BS acts as considerable interference to the UL signal.
To address the interference in the CTS, a Time Slot Allocation (TSA) scheme, a power control scheme, a sectored antenna direction adjustment scheme, and an antenna tilting scheme have been suggested. The TSA scheme avoids allocating UL or DL resource to the CTS, or does not allocate the CTS by allocating DL resources from the left to the right in the frames and allocating UL resources from the right to the left in the frames. The power control scheme regulates the DL transmit power of the BS or the UL transmit power of the MS in the CTS. The antenna direction adjustment scheme varies the antenna directions of the neighbor BSs. The antenna tilting scheme regulates the interference between the neighbor BSs by adjusting the tilt angle of the antenna.
However, when the cell load increases and thus considerable interference of the neighbor BS is caused while allocating all UL resources of the CTS, the STA scheme may fail to receive the UL signal in the CTS. While the interference of the neighbor BS is not so great, when the MSs are traveling mainly in the cell boundary, the STA scheme may fail to receive the UL signal. Moreover, since the STA scheme allocates the resources by taking into account the interference effect on the MS, the MS position, and the cell capacity, the complexity of the scheduler is likely to increase. The power control scheme and the antenna direction adjustment scheme are subject to the performance change depending on the MS distribution. The antenna tilting scheme can be performed in the initial cell configuration. When adjusting the tilt angle of the antenna, the cell coverage may decrease.